As one of the conventional seat slide devices, there is proposed a seat slide device, which includes a lower rail fixed to a vehicle floor and an upper rail fixed to a vehicle seat and slidably supported by the lower rail. The lower and upper rails are relatively slidable in a vehicle longitudinal direction by driving a motor so that the forward and backward positions of the vehicle seat can be adjusted.
In this type seat slide device, the lower rail is fixed to the vehicle floor by means of nut member and a screw shaft is engaged with the nut member. The screw shaft extends in a longitudinal direction of the rails and is rotatably supported on the upper rail. The rotational force is transmitted from the motor to the screw shaft through a gear device provided on a front end of the upper rail. This structure causes a slidable movement of the upper rail relative to the lower rail by rotating the screw shaft by the rotational force from the motor.
In the conventional device, upon a vehicle collision accident, such as an rear-end collision, a vehicle seat receives a large load in front and backward direction due to the inertia caused by the vehicle seat itself and an occupant thereon by the impact of the collision. This large load is transmitted from the upper rail which supports the vehicle seat to the lower rail through the gear device provided on the front end of the upper rail, the screw shaft connected to the gear device and the nut member engaged with the screw shaft. The gear device mainly receives the load due to the structural reasons. This disadvantage is one of the issues to be improved and various improvements therefor have been proposed to avoid such load concentration on the gear device.
For example, a power seat device proposed in the Patent Document 1 listed below is provided with an accommodation portion for accommodating the gear case at the front end of the upper rail. The gear case is accommodated in the accommodation portion by a support bracket to fixedly support the gear case in front and backward directions so that the bracket receives the load applied on the gear case.
Another improvement is proposed in the Patent Document 2. In this device both sidewalls of the upper rail are bent inward with a predetermined width to form a concave portion and a pair of washers are provided at the screw shaft to be in contact with the walls of the concave portion in a front and backward direction. The load applied on the gear case is received by the two washers, which are in contact with the concave portion of the upper rail.    Patent Document 1: Japanese utility model 7-345    Patent Document 2: Japanese Patent 2002-192996 A
The device in the Patent Document 1, since the front end of the upper rail has to be extended and to be swollen to form the accommodation portion. This needs a press machining process instead of a simple roll forming. The press machining requires a very high degree of accuracy not to influence upon the accuracy of flatness of the sliding surface with the lower rail. In addition, the weight of the upper rail may increase which eventually leads to the increase of manufacturing cost.
In the device in the Patent Document 2, in order to secure a space between the washers, a collar member has to be inserted between the washers, which leads to the increase of the number of parts and assembling process. In addition, a small clearance between the washers and the concave portion is needed which requires a high degree of accuracy. These will lead to the increase of manufacturing cost. The upper rail has to be formed by press machining as like the upper rail in the Patent Document 1.
The present invention has been made in view of the above circumstances and an object of the invention is to reduce the concentration of the load applied to the gear device when a large load is applied thereto such as, upon a vehicle collision, to tightly hold the gear device.